1. Field of the Invention
The present invention relates to a positive electrode active material having a layered rock-salt structure for lithium secondary batteries, and a positive electrode including the positive electrode active material.
2. Description of the Related Art
Positive electrode active materials including a lithium complex oxide (lithium transition metal oxide) having a layered rock-salt structure are widely known as positive electrode active materials in lithium secondary batteries (which may be referred to as lithium ion secondary batteries) (see, for example, Patent Document 1 (JP5-226004A) and Patent Document 2 (JP2003-132887A)).
It is known that, in a positive electrode active material of this type, diffusion of lithium ions (Li+) therein is performed along an in-plane direction of the (003) plane (i.e., any direction in the plane parallel to the (003) plane), while intercalation and deintercalation of lithium ions occur through a crystal plane other than the (003) plane (e.g., the (101) plane or the (104) plane).
Thus, in positive electrode active materials of this type, attempts have been made to enhance battery characteristics of lithium secondary batteries by exposing a crystal plane through which lithium ions are favorably intercalated and deintercalated (i.e., a plane other than the (003) plane, such as the (101) plane or the (104) plane) to a surface which comes into contact with a larger amount of electrolyte (see, for example, Patent Document 3 (WO2010/074304)).
In addition, in positive electrode active materials of this type, those having pores (also referred to as holes or voids) formed therein are known (see, for example, Patent Document 4 (JP2002-75365A), Patent Document 5 (JP2004-083388A), and Patent Document 6 (JP2009-117241A)).
In particular, Patent Document 7 (WO2012/137391) discloses a lithium secondary battery positive electrode active material with a layered rock-salt structure, comprising secondary particles composed of numerous primary particles having an average particle diameter of 0.01 μm or greater and 5 μm or less, wherein the secondary particles have a degree of (003) plane orientation of 60% or greater, an average particle diameter of 1 μm or greater and 100 μm or less, an aspect ratio, which is a value obtained by dividing the longer axis diameter by the shorter axis diameter, of 1.0 or greater and less than 2, a voidage of 3% or greater and 30% or less, an average pore diameter of 0.1 μm or greater and 5 μm or less, and a value obtained by dividing the average particle diameter of the primary particles by the average pore diameter of 0.1 or greater and 5 or less.
In addition, a positive electrode active material having through holes and hollow parts is also known. For example, Patent Document 8 (WO2012/153379) discloses positive electrode active material particles having a shell part composed of a lithium transition metal oxide, a hollow part formed in the interior of the shell, and through holes penetrating the shell, wherein the hollow part accounts for 23% or greater of the apparent cross-sectional area, and the shell part has a thickness of 2.2 μm or less. Patent Document 9 (JP2004-253174) discloses a positive electrode active material for non-aqueous electrolyte secondary batteries, wherein the lithium transition metal complex oxide is a lithium transition metal complex oxide composed of hollow particles having an outer shell part on the outside and a space part inside the outer shell part, and when the cross-section is revealed, the proportion of the area of the space part to the total of the outer shell part and the space part is preferably greater than 0% and less than 20% and is optimally 7 to 16%.